A fuel cell has been widely used as a clean power generation system for little exerting negative influence on the environment. That is because a fuel cell generates electric power using hydrogen and oxygen as reaction gases to produce only water as a product in the electrode reaction. Currently, main application of a fuel cell spreads to various devices such as a fuel cell vehicle, a fuel cell for home use, and a mobile device.
Among many types of fuel cells, a polymer electrolyte fuel cell (PEFC) works at the operational temperature from room temperature to a degree of 90° C., and may rapidly generate electric power, allowing the PEFC to be widely used for a fuel cell vehicle.
Internal surroundings of the polymer electrolyte fuel cell used for the fuel cell vehicle, especially, units in a vicinity of electrodes (hereinafter, those may be collectively called a proton conductive membrane, an electrode catalyst layer and a gas diffusion layer (see FIG. 3)) are influenced by the frequent operations of starting and stopping of the fuel cell vehicle. Further, the units turn into an excessively dry state (or a highly dry state) or an excessively humid state (or a highly humid state) resulting from various changes in a driving environment and situation. This may cause changes in the power generation surroundings.
The vicinity of electrodes, for example, becomes a highly dry state when a polymer electrolyte fuel cell has just started, or a humidifier does not catch up with electric power generation or the like. Then, after the polymer electrolyte fuel cell has been operated for awhile after the starting, the electrochemical reaction proceeds to produce water in an electrode catalyst layer, leading to a highly humid state.
Note if the polymer electrolyte fuel cell continues power generation in the highly dry state, ionic conductivity of the proton conductive membrane (or electrolytic membrane) or the like decreases thereby to increase the ionic conductive resistance of the membrane, resulting in deterioration in the power generation performance. On the other hand, if the polymer electrolyte fuel cell is placed in a highly humid state, micropores in the gas diffusion layer and the electrode catalyst layer are choked thereby to inhibit a flow of the reaction gases, resulting in deterioration in the power generation performance.
Accordingly, the change in the humidity condition in the vicinity of the electrodes may largely influence the power generation performance of the polymer electrolyte fuel cell. For example, this may make the power generation performance unstable. Such a drawback causes a strong demand for reducing the influence resulting from the change in the humidity condition.
Here, Patent Documents 1 to 3 disclose techniques for reducing the influence resulted from the change in the humidity condition with respect to polymer electrolyte fuel cells.
As shown in FIG. 6, Patent Document 1 discloses a membrane electrode structure for a polymer electrolyte fuel cell 610 comprising: a cathode (or an air electrode) 620 having a catalyst layer 622, an intermediate layer 624 and a gas diffusion layer 626 in the order; an anode (or a fuel electrode) 630 having a catalyst layer 632, an intermediate layer 634 and an a gas diffusion layer 636 in the order; a polymer electrolyte membrane 640 corresponding to a proton conductive membrane 2 of the present invention (see FIG. 3) arranged between the catalyst layer 622 of the cathode 620 and the catalyst layer 632 of the anode 630. Further, at least either of the catalyst layers 622 and 632 of the cathode 620 and the anode 630 includes a catalyst containing platinum, and the intermediate layers 624 and 634 contain carbon fibers. Herein, at least either of the intermediate layers 624 and 634 needs to satisfy at least one of the following conditions: (a) the platinum rate contained in the intermediate layer is 3-20 at % to the 100 at % of platinum contained in both the catalyst layer and the intermediate layer, and (b) 90 at % or more of platinum contained in the intermediate layer exists in the region up to a half of the thickness of the intermediate layer from the interface between the catalyst layer and the intermediate layer. Moreover, Patent Document 1 describes that preferably platinum is a supported catalyst in which platinum or a platinum alloy is supported on a carbon carrier.
Furthermore, Patent Document 1 describes not only that the contained carbon fibers form conductive paths to improve the electron conductivity but also that the capillary action makes the produced water (or water vapor) rapidly move from the catalyst layers 622 and 632 to the intermediate layers 624 and 634.
Further Patent Document 2 discloses as shown in FIG. 7, a membrane electrode assembly 712 for a fuel cell comprising: a cathode catalyst layer 716, a cathode backing layer 717 and a cathode diffusion layer 718 stacked at the cathode side of an electrolyte membrane 715 corresponding to a proton conductive membrane 2 of the present invention (see FIG. 3 in this specification); an oxygen gas passage 724 arranged at the outside of a cathode diffusion layer 718; an anode catalyst layer 721, an anode backing layer 722 and an anode diffusion layer 723 stacked at the anode side of the electrolyte membrane 715; and a hydrogen gas passage 725 arranged outside an anode diffusion layer 723. Here, each above mentioned layer and the electrolyte membrane 715 are arranged in the direction of the gravity. Further, Patent Document 2 describes that a water adsorbent is added to the cathode backing layer 717 and specifically NAFION® (registered trademark) is used as a water absorbent, which also works as an adhesive agent.
Patent Document 3 discloses fuel cell electrodes for a fuel cell 810 as shown in FIG. 8 including a fuel electrode (or an anode) 808 having a catalyst layer 804 at a fuel electrode side and a diffusion layer 805, together with an air electrode (or a cathode) 817. Further, Patent Document 3 discloses the air electrode 817 comprising: an electrolyte layer 802 corresponding to the proton conductive membrane 2 of the present invention (see FIG. 3 in this specification), a first catalyst layer 813a, a second catalyst layer 813b, and a diffusion layer 805 in the order. Herein, the first catalyst layer 813a is larger resistant to gas movement than the second catalyst layer 813b. Accordingly, the first catalyst layer 813a prevents hydrogen which has passed through the electrolyte membrane 802 from further moving, thereby to facilitate the hydrogen to be oxidized. The first catalyst layer 813a includes a carbon supported Pt catalyst, and the second catalyst layer 813b includes a Pt-Black catalyst (not shown in FIG. 8) corresponding to noble metallic nanoparticles 51 (see FIG. 3 in this specification). The Pt-Black catalyst has properties of a high affinity with water and a small contact angle with water, allowing a large amount of water to physically adhere to a surface of the catalyst.